1. Field of the Invention
The present invention relates to a welding start position detecting apparatus for a welding robot.
2. Description of the Related Art
An ordinary welding robot has a function of correcting a shift between an actual welding start position and a target welding start position.
FIG. 4 shows a prior art arrangement of such an apparatus in which, in a welding start position detecting operation, a welding start position detecting voltage V generated at a start position detecting voltage generating circuit 1 is applied to a welding wire 2, under which condition a robot is driven to bring a tip end of the welding wire into contact with a welding start position on a base material 3.
The voltage V applied to the welding wire 2 is sampled and applied to a terminal A of a comparison circuit 4. The comparison circuit 4 compares the applied voltage V with a predetermined reference voltage Vr received from a reference voltage generating circuit 5 and outputs a start position detection signal when the voltage V becomes lower than the reference voltage Vr.
Since the base material 3 is grounded, when the welding wire 2 comes into contact with the base material 3, the voltage signal V applied to the comparison circuit 4 becomes zero. The reference voltage Vr is previously set to a value between the voltage generated when the welding wire 2 comes into contact with the base material 3 and the voltage when the welding wire 2 does not come into contact with the base material 3. In the normal case, when the welding wire 2 comes into contact with the base material 3, the voltage signal V becomes lower than the reference voltage, which results in that the comparison circuit 4 outputs a start position detection signal.
A robot controller 6 stops the robot when receiving the detection signal from the comparison circuit 4, calculates on the basis of angles of respective axes of the robot a welding start position at which the welding wire 2 contacts with the base material 3, compares the calculated value with a target welding start position previously set to find a shift therebetween, and corrects the position of the robot according to the calculated shift.
With such a prior art system as mentioned above, since an impedance around a welding torch largely varies due to the adhesion of spatter, the leakage of cooling water, the reduction of insulation of the cooling water, etc., even when the identical voltage is generated from the detecting-voltage generation circuit 1, the voltage V applied to the comparison circuit 4 is varied.
For this reason, the prior art system has such a disadvantage that, when the detection voltage V is largely lowered, the comparison circuit 4 outputs the detection signal in spite of the fact that the welding wire 2 does not come into contact with the base material 3, whereby it becomes impossible to distinguish whether the welding start position is actually detected or not.
It is an object of the present invention to provide an apparatus for detecting a welding start position in a welding robot which, even when an impedance around a welding torch changes, can maximize the possibility of detecting a welding start position on a base material so as to improve the working efficiency of a welding robot.